1. Field of the Invention
The present invention relates to the detection of high speed events and more particularly relates to detecting and locating photon emitting high speed events using a charge coupled device focal plane array.
2. Description of the Prior Art
High speed events have traditionally been detected with gated image tubes, camera tubes and streak camers which utilize charge coupled devices (CCD). It is also known that high speed events can be detected using a cueing sensor, a focal plane array and an optical delay line wherein the cueing sensor is coupled to a gate of the focal plane array in order to control the current state of the focal plane array. The gate is used to hold the focal plane array in a non-integrating (non-charging or drain) state until the cueing sensor detects a photon emitting high speed event. Upon detection of such an event, the gate of the focal plane array receives an electrical signal from the cueing sensor which switches the focal plane array to a second state called the integrate (charging) state. The optical delay line is utilized to delay presentation of the high speed event to the focal plane array until the focal plane array is switched from the non-integrating state to the integrate state. In order for the system to operate properly, the optical delay line must provide a delay at least equal to the sum of the cueing sensor response time and the focal plane array non-integrate (drain) to integrate (charging) switching time before the photons from the high speed event are presented to the focal plane array. As a result, the focal plane array will become active before the photons from the high speed event encounter the focal plane array. Therefore, the focal plane array is capable of recording and integrating the high speed event.
Using an optical delay line to assist in the recognition and delay the presentation of high speed events provides a relatively high probability of detecting an event. However, this method is inadequate in several respects. Fiber optic bundles are generally bulky, costly and, have significant optical attenuation on the order of approximately 20 dB which significantly effects the probability of detecting a high speed event. The optical attenuation is attributed to the length of fiber required to achieve the necessary delay for proper operation. Furthermore, the fiber optic bundles often degrade the image quality presented to the focal plane array. Broken fibers and dead zones due to the inefficient packing of the fibers introduce voids into the field of vision, which although small, can reduce the probability of detecting a photon emitting high speed event.
Standard high speed event detection systems using CCD's integrate scene photon radiation approximately 60 times/sec (16 msec intervals) with integration times of 1.0 msec to 16.7 msec with a low probability of intercept (i.e., at 1.0 msec). Due to this relatively long integration time, a significant amount of background light may accumulate on the focal plane array which can clutter a scene with unwanted charge. As a result, it is often difficult to detect a fast temporal photon emitting source from the unwanted charge which has been allowed to accumulate or integrate on the focal plane array.